Evaporative cooling systems are utilized in such processing equipment as condensers, coils, jackets, as well as other heat exchangers. Evaporative cooling systems operate on the principle that the latent heat of vaporization of water being evaporated removes energy from the system, thus, reducing the temperature of the remaining water in the system. In particular, cooling towers are widely used in industry to transfer process waste heat to the atmosphere. In the interests of economy, the aqueous coolant used in these systems is generally recycled.
In cooling towers, the warmed coolant is generally permitted to flow over a large surface that is subjected to a draft of air, either forced or natural, to bring about evaporation of a portion of the exposed coolant. The remaining coolant, which has given up heat to supply the heat of vaporization of the portion evaporated, flows to a reservoir from which it is pumped back to the processing equipment for the absorption of more heat, thus completing the cycle. During this process, the coolant can absorb oxygen from the air which adds to its corrosiveness. Additionally, as some of the coolant is evaporated, the salts and other impurities remaining in the coolant are concentrated. This results in an increased amount of dissolved solids in the recirculating stream.
The most common dissolved salts in domestic water, used as a coolant, are bicarbonates, chlorides, and sulfates of calcium, magnesium, and sodium. When water containing calcium bicarbonate is heated, as in cooling of air conditioning systems or other equipment, the heat will strip off one molecule of carbon dioxide, converting the remaining calcium salt to calcium carbonate (limestone) according to the equation:Ca[HCO3]2+[heat]→CaCO3↓+CO2+H2O
Typically, various additives, such as corrosion inhibitors, anti-fouling agents, and microbiocides are mixed with the water. These additives serve to minimize corrosion of the equipment being served and to maintain the equipment within practical limits of efficiency, by minimizing the formation of scale (such as the calcium carbonate discussed above), sludge deposits and biological growth. Other additives may also include acids, such as sulfuric acid, which may be introduced as required to maintain a desired pH of the coolant, generally between about 6.3 and 7.5. Too low a pH will lead to corrosion, whereas too high a pH in the presence of hard water results in scale and other deposits on the water side of the processing equipment.
In the past, it had frequently been the practice to shut down a cooling system after six to twelve months of operation to clean waterside surfaces of the processing equipment being served. In view of high labor costs and other considerations, the tendency more recently has been to run for longer periods of time, such as 24 to 36 months, before having to shut down. For this reason, scale and other deposits on the waterside surfaces are a major consideration, especially since the presence of appreciable scaling and fouling limits the efficiency of the processing equipment. Where peak efficiency is required, scaling and fouling become of primary concern. Deposits on waterside surfaces mean reduced and frequently uneven heat transfer, poor corrosion inhibitor performance, shortened equipment life, increased pumping costs and product loss due to ineffectual cooling.
Contemporary cooling tower systems tend to concentrate the hardness of the aqueous coolant and other contained undesirable impurities because of the rapid evaporation which is characteristic of all such cooling towers. If no steps were taken to rid the systems of this unwanted material and to limit the degree of hardness of the circulating coolant, the processing equipment would be fouled very rapidly. It would be rendered inefficient, and would require frequent down time for cleaning and actual equipment replacement. In an attempt to overcome these difficulties, a procedure is used known as “blowdown” or “bleed,” in which a certain percentage of the recirculating coolant stream is purged from the system, carrying with it a portion of the unwanted scale and deposit-forming impurities. The blowdown is generally based on maintaining a materials balance in the system, so that the scaling and fouling constituents are not sufficiently concentrated to result in deposition on heat transfer surfaces.
The required amount of blowdown can be quite considerable. For example, with a typical, moderately sized unit having a rate of circulation of 5,000 gallons per minute (gpm), the total quantity of blowdown over a 24 hour period can amount to 72,000 gallons, or more than three times the total content of the system. This discarded water represents a very appreciable loss, both monetarily, and as a valuable resource. The blowdown unfortunately also carries with it the contained additives. The loss of water and valuable additives for a moderate sized industrial unit may amount to many thousands of dollars per year of operation.
Therefore, there is a need in the art for a system for maintaining cooling water or other coolant fluids in proper operating conditions, which are consistent with both resource usage and cost of operation.